The present invention relates generally to surge arresters and more particularly to a surge arrester of the gapless type.
Recent research in surge arresters has demonstrated that zinc oxide has the capability of providing a low cost "gapless" arrester as a result of its relatively low power dissipation under steady-state conditions coupled with its ability to clamp voltage at large currents. However, experiments have shown that for a given zinc oxide process the selection of its steady-state voltage rating involves a compromise between thermal runaway and the desire to have an operating voltage close to cross-over. Moreover, it has been noted that a relatively small amount of power, on the order of about 15 watts, is sufficient to cause thermal runaway for certain zinc oxide arresters.
From the foregoing, it should be apparent that gapless surge arresters must be designed with heat dissipation in mind, particularly when the surge arrester is used outdoors and requires a protective casing. A typical gapless surge arrester of this type includes a porcelain outer casing and a stack of zinc oxide discs within the casing for passing surge currents therethrough. In this typical surge arrester, a layer of air (or nitrogen) is maintained between the zinc oxide discs and porcelain casing and hence must act in conjunction with the casing to dissipate the heat generated in the discs as a result of surge currents therethrough. While this is a practical and economical way to dissipate heat it is not highly effective and hence requires a relatively large safety margin between the operating voltage of the arrester and its cross-over to prevent thermal runaway.
There are ways to transfer the heat generated in the zinc oxide discs to the outer procelain casing other than by air or nitrogen. For example, oil or freon could be used and would be more effective than providing an air gap. However, both the oil and freon cause internal pressure problems and, in addition, the freon is relatively expensive. On the other hand, as will be seen hereinafter, the present invention is directed to the utilization of a material which is both practical and economical and yet one which is more effective than air and even oils. Moreover, the particular material selected has additional benefits as will also be seen hereinafter.
Objects and Summary of the Invention
One object of the present invention is to provide a gapless surge arrester designed to effectively and efficiently dissipate heat during current surges to permit operation of the arrester closer to its cross-over point without the fear of thermal runaway.
Another object of the present invention is to provide effective and efficient heat dissipation from both practical and economical standpoints.
Still another object of the present invention is to provide a gapless surge arrester which is designed to minimize damage to its outer casing as a result of excessive internal fault energy.
Yet another object of the present invention is to provide a method of dissipating heat from inside the arrester without interfering with the necessary physical movement of its inner components.
A gapless surge arrester of the type to which the present invention is directed typically includes an open ended electrically non-conductive but thermally conductive outer casing, typically one constructed of porcelain, having an inner wall defining an opening therethrough. This surge arrester also includes means extending through the opening and spaced from the inner wall, typically a stack of zinc oxide or other such metal oxide discs, for passing surge currents. However, rather than maintaining an air gap between this stack of discs and the outer casing and rather than providing oil or freon therebetween, the present invention utilizes an electrically non-conductive particulate material, particularly silicon dioxide (preferably sand). As will be seen hereinafter, this particular material has been found to be more effective and efficient in transferring heat across the gap then air and even oil and is substantially similar to freon. Moreover, it has been found to absorb fault energy by changing to glass and cinders, thereby reducing the serverity and intensity of operation of the surge arrester and reducing the possibility of damage to its casing. In addition, the particulate material allows the discs to expand and contract and otherwise move to a limited degree within the casing.